1. Field of the Invention
The invention relates to a control apparatus of an inverter and a power generation system using such a control apparatus. More particularly, the invention relates to a control apparatus of a system linkage inverter which is linked to a commercially available power source system and a power generation system using such a control apparatus and capable of coping with an unstable power generation such as a solar power generation.
2. Related Background Art
A solar power generation system that converts an inexhaustible clean solar energy into an electric power is desirable. Particularly, in recent years, legal preparation is progressing and a system linkage system for converting a DC electric power which is generated by a solar cell into an AC power by using an inverter and for supplying the AC power to the commercially available power source system can be used on a full scale. FIG. 1 shows an example of a general solar power generation system. An output of a solar cell 1 is connected to a commercially available power source system 3 through a system linkage inverter (hereinafter, simply referred to as an inverter) 2. The inverter 2 has therein: an input filter (DC side filter) 21 having a coil and a capacitor; a switching unit 22 having a semiconductor switch which is on/off controlled by a gate control signal and the like; an output filter (AC side filter) 23 having a coil and a capacitor; and a control apparatus 24 for controlling the operation of an inverter. The control apparatus 24 has a maximum power control unit 241 and an instant value current control unit 242. The inverter 2 can also have a protecting apparatus and the like.
The maximum power control unit 241 changes an operation point of a solar cell in accordance with a change in insolation intensity or temperature, thereby extracting the maximum electric power from the solar cell. The control unit 241 inputs a solar cell voltage Vpv and a solar cell current Ipv and arithmetically operates an output current instruction value of the inverter so that a solar cell output becomes maximum. More particularly, the control unit 241 arithmetically operates and calculates a voltage such that the solar cell electric power becomes maximum and controls the current instruction value so that the solar cell voltage is equal to such a voltage. It is a general way to use a digital microprocessor (what is called a micom) as an arithmetic operating apparatus. As an example of such a maximum power control unit, there is a unit as disclosed in Japanese Patent Application Laid-open No. 62-85312.
The instant value current control unit 242 inputs the current instruction value from the maximum power control unit 241 and controls the switching unit 22 by a gate control signal so that an output current of the inverter almost coincides with the current instruction value. As an example of such an instant value current control unit, there is a unit as disclosed in U.S. Pat. No. 4,424,557.
An example of the operation of the instant value current control unit will now be described with reference to FIGS. 1 and 2. The instant value current control unit 242 inputs the current instruction value from the maximum power control unit 241 and sends a gate control signal (on/off) instruction signal to the switching unit 22. FIG. 2 shows relations among the gate signal, output current waveform, and current instruction value in this instance. As shown in FIG. 2, as for the instant value current control signal, a gate control signal is transmitted so that the output current almost coincides with the current instruction value. Namely, when the gate signal is at the high (H) level, the output current increases. When the output current exceeds the instruction value by a predetermined amount or more, the gate signal is set to the low (L) level, thereby reducing the output current. When the output current is lower than the instruction value by a predetermined amount or more, the gate signal is again set to the H level, thereby increasing the output current. By such a control operation, the output current is almost made coincide with the instruction value. In the inverter for solar power generation system, in order to reduce noises and an output current distortion, in many cases, the highest frequency of the gate signal is set to a fairly high value in a range from about 10 kHz to 30 kHz.
The maximum power control unit and instant value current control unit are ordinarily individually constructed. This is because control speeds which are required for the maximum power control and the instant value current control are quite different. Since it is sufficient that the maximum power control unit merely traces a fluctuation of the insolation, a control unit having a lower switching speed can be used. According to the study of the inventors et al., it is sufficient that the control unit can operate at a slow period such as 0.1 second. However, the instant value current control system continuously compares the output current with the current instruction value and must decide an on/off of the gate from the comparison result. Moreover, in order to reduce an output current distortion and noises, it is required to turn on/off the switching unit at a frequency of 10 kHz or higher. For example, in order to realize a switching speed of 10 kHz, it is necessary to suppress a control period to a value less than at most 100 microseconds. If both of the control operations are executed by one digital microprocessor, all of control logics can be assembled as a software, so that a flexibility of the system remarkably rises. However, if such a construction is ordinarily embodied, both of the maximum power control and the instant value current control are executed in the control period (for example, 100 .mu.sec), so that an extremely high speed microprocessor and an extremely high speed analog/digital converter are necessary. They are very expensive and there is a problem such that the inverter cannot be cheaply constructed. Therefore, in general, the instant value current control unit constructed like a hardware and the maximum power control unit constructed like a software are combined as mentioned above. Although it is easy to perform the instant value current control by the hardware, since two control units of the hardware and the digital microprocessor are provided, the costs rise and an installing place is necessary. When a new switching device appears, it is necessary to change the design of the hardware of the current control unit.
An inverter control apparatus using an interruption by a timer has been disclosed in Japanese Patent Application Laid-open No. 57-25171. However, such an apparatus cannot be applied to an inverter having an input such that it successively changes in accordance with an insolation like a solar cell.
As mentioned above, a proper control apparatus suitable for a linkage inverter to perform the instant value current control (waveform formation control) and the maximum power control by using a single CPU is not yet realized.